#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;

int main(){
    int n,d;
    cin >> n >> d;
    vector<int> t(n),k(n);
    rep(i,n) cin >> t[i] >> k[i];
    vector<vector<int>> dp(n+1,vector<int>(2,0));
    dp[0][0] = 0;
    dp[0][1] = -d;
    for(int i=1;i<=n;i++){
        dp[i][0] = max(dp[i-1][0]+t[i-1],dp[i-1][1]-d+k[i-1]);
        dp[i][1] = max(dp[i-1][0]-d+t[i-1],dp[i-1][1]+k[i-1]);
    }
    cout << max(dp[n][0],dp[n][1]) << endl;
    return 0;
}